(1) Field of the Invention
The invention generally relates to a semiconductor wafer carrier and, more particularly to methods of improving the apparatus used in holding the wafer during the polishing process.
(2) Description of Prior Art
Semiconductor fabrication often uses a combination of chemical and mechanical polishing to reduce the thickness and planarize a thin film coating on a wafer. Typically, the wafer is placed in a polishing head and makes contact with a rotating polishing pad having a slurry applied thereto. Often the polishing head holding the wafer also rotates making the planarization process more uniform.
FIGS. 1 and 2 schematically show a cross section of the current art for the polishing process. The wafer 14 is held in place laterally by the guide rings 20. To facilitate thin film planarization, uniform pressure is applied mechanically from above to the carrier 18 holding the wafer 14 firmly against the polishing pad 12. To aid in maintaining uniform pressure to the wafer 14, a thin carrier film 16 is usually attached to the carrier 18. The polishing table 10 and polishing pad 12 are rotated at a set speed, while often, the carrier 18, carrier film 16, and wafer 14 rotate at a second set speed. During automated loading and unloading, the wafer is held onto the carrier by vacuum pressure via passages 22.
The current practice uses plastic or metal shims to set the gap between the guide ring and carrier. This ensures that the wafer stays under the carrier during chemical mechanical polishing (CMP). The shim thickness is not adjustable around the circumference of the guide ring and because of variation in the shim thickness and uneven wear rate on the guide ring, non-uniform pressure may be applied to the wafer. This compromises the process quality by unevenly removing the thin film material during CMP. Operating cost also increase since the guide ring must be reconditioned or discarded when it no longer meets specifications.
A vacuum is used to remove the wafer from the polishing table after completing the CMP process. During this removal process, the vacuum may also draw polishing slurry into the point of contact between the carrier and guide ring. Slurry in this area will cause the guide ring to be out of tolerance, a problem that is exacerbated if the slurry is permitted to dry. Since the slurry does not evenly fill the gap, this also inhibits uniformity of pressure applied during wafer polishing.
Other approaches attempt to address problems in maintaining uniform pressure across the surface of the wafer during polishing. U.S. Pat. No. 5,681,215 to Sherwood et al. teaches a method using multiple bellows forming two pressure chambers. One chamber is used to apply an even load across the wafer and the other is used to press the retaining ring and wafer against the polishing pad. U.S. Pat. No. 5,876,273 to Yano et al teaches a method using a pressure-absorbing member between the carrier and guide ring. This member allows movement of the guide ring with respect to the carrier while maintaining uniform pressure on the wafer. U.S. Pat. No. 5,584,751 to Kobayashi et al teaches a method whereby pressure is applied to a diaphragm allowing the position of the wafer and carrier to be adjusted during the CMP process. U.S. Pat. No. 5,423,716 to Strasbaugh teaches a method of holding the wafer during loading and unloading using negative pressure on a flexible membrane. This creates small suction cups in the membrane, holding the wafer in place. By applying positive pressure to the membrane, the wafer can be released, or, during CMP, held with uniform pressure against the polishing pad. U.S. Pat. No. 5,851,140 to Barns et al. teaches a method using a flexible carrier plate providing an air pillow that maintains uniform pressure on the wafer during CMP.